Network packet

Source: Wikipedia, the free encyclopedia.

In

error detection codes, or sequencing information). Typically, control information is found in packet headers and trailers
.

In

bit stream
.

Terminology

In the seven-layer

TCP segment is carried in one or more IP packets, which are each carried in one or more Ethernet frames
.

Architecture

The basis of the packet concept is the postal letter: the header is like the envelope, the payload is the entire content inside the envelope, and the footer would be your signature at the bottom.[3]

Network design can achieve two major results by using packets: error detection and multiple host addressing.[4]

Framing

Communications protocols use various conventions for distinguishing the elements of a packet and for formatting the user data. For example, in Point-to-Point Protocol, the packet is formatted in 8-bit bytes, and special characters are used to delimit elements. Other protocols, like Ethernet, establish the start of the header and data elements by their location relative to the start of the packet. Some protocols format the information at a bit level instead of a byte
level.

Contents

A packet may contain any of the following components:

Addresses
The routing of network packets requires two network addresses, the source address of the sending host, and the destination address of the receiving host.
Error detection and correction
Error detection and correction is performed at various layers in the protocol stack. Network packets may contain a checksum, parity bits or cyclic redundancy checks to detect errors that occur during transmission.
At the transmitter, the calculation is performed before the packet is sent. When received at the destination, the checksum is recalculated, and compared with the one in the packet. If discrepancies are found, the packet may be corrected or discarded. Any packet loss due to these discards is dealt with by the network protocol.
In some cases, modifications of the network packet may be necessary while routing, in which cases checksums are recalculated.
Hop limit
Under fault conditions, packets can end up traversing a
network hop
. If the field reaches zero, routing has failed, and the packet is discarded.
Ethernet packets have no time-to-live field and so are subject to
broadcast radiation in the presence of a switching loop
.
Length
There may be a field to identify the overall packet length. However, in some types of networks, the length is implied by the duration of the transmission.
Protocol identifier
It is often desirable to carry multiple communication protocols on a network. A protocol identifier field specifies a packet's protocol and allows the protocol stack to process many types of packets.
Priority
Some networks implement quality of service which can prioritize some types of packets above others. This field indicates which packet queue should be used; a high priority queue is emptied more quickly than lower-priority queues at points in the network where congestion is occurring.
Payload
In general, the payload is the data that is carried on behalf of an application. It is usually of variable length, up to a maximum that is set by the network protocol and sometimes the equipment on the route. When necessary, some networks can break a larger packet into smaller packets.

Examples

Internet protocol

IP packets are composed of a header and payload. The header consists of fixed and optional fields. The payload appears immediately after the header. An IP packet has no trailer. However, an IP packet is often carried as the payload inside an Ethernet frame, which has its own header and trailer.

Per the

transport protocol such as Transmission Control Protocol
on top of the packet service to provide such protection.

NASA Deep Space Network

The Consultative Committee for Space Data Systems (

CCSDS
) packet telemetry standard defines the protocol used for the transmission of spacecraft instrument data over the deep-space channel. Under this standard, an image or other data sent from a spacecraft instrument is transmitted using one or more packets.

MPEG packetized stream

elementary stream
to be divided into packets. The elementary stream is packetized by encapsulating sequential data bytes from the elementary stream between PES packet headers.

A typical method of transmitting elementary stream data from a video or audio encoder is to first create PES packets from the elementary stream data and then to encapsulate these PES packets inside an

DVB
.

NICAM

In order to provide mono compatibility, the NICAM signal is transmitted on a subcarrier alongside the sound carrier. This means that the FM or AM regular mono sound carrier is left alone for reception by monaural receivers. The NICAM packet (except for the header) is scrambled with a nine-bit pseudo-random bit-generator before transmission. Making the NICAM bitstream look more like white noise is important because this reduces signal patterning on adjacent TV channels.

See also

References

  1. ISBN 0-13-088263-1
    . Packet: A group of bits that includes data plus control information. Generally refers to a network layer (OSI layer 3) protocol data unit.
  2. ^ "OSI Model".
  3. ^ "Understanding The OSI Reference Model: An Analogy", The TCP/IP Guide, archived from the original on 2014-08-09, retrieved 2014-08-09
  4. ^ "Chapter 5 Link Layer". www.msc.uky.edu. Retrieved 2021-10-23.
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